1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, an electronic parts mounting method and heating/melting process equipment and, more particularly, to a semiconductor device manufacturing method containing a solder bump forming step, an electronic parts mounting method for mounting electronic parts on a substrate, etc., and heating/melting process equipment employed in these methods.
2. Description of the Prior Art
As the method of forming the solder bump on the semiconductor substrate on which the semiconductor device is formed, there are the plating method, the printing method, the solder ball putting/melting method, etc. Then, the solder bump formed on the semiconductor substrate is melted and jointed to the connected material such as the wirings, etc. In case the solder bump is connected to the terminal of the wiring substrate, normally the method that melts the solder bump as well as cleans the terminal surface by removing the oxidized layer from the surface using flux is employed.
If flux is employed, the soldering step is performed via following processes.
First, when the flux that is coated on the deposited surface of the terminal, the wiring, etc. is heated, such flux can cover the deposited surface while activating the surface to prevent new oxidation, whereby the active state of the deposited surface can be maintained. Also, when the solder is melted on the deposited surface, such solder spreads over the deposited surface and dissolves a part of the flux. In addition, when the solder is cooled on the deposited surface, such solder is solidified and jointed to the deposited surface, so that the remaining flux and the dissolved product are solidified.
As the first one of the problems caused in such steps, there is the problem due to the removal of the flux.
In other words, in order to clean the deposited surface, the method of removing the solidified flux by using the organic solvent other than flon and Trichlene is employed. In this case, since the dissolved product cannot be simply removed by a small amount of organic solvent, a large amount of organic solvent must be consumed.
The use of flon and Trichlene has already been inhibited from viewpoints of ozone depletion prevention and ground water pollution prevention. Also, since other organic solvents have a harmful effect on the environment, it is desired to develop a solder bump forming method which does not need cleaning.
Also, as the second one of the problems, it may be considered that the voids are formed in the inside of the solder bump in the middle of the solder melting. The voids generated at this time are still remained in the solder bump after the semiconductor device has been mounted, and thus the reliability of connection between the solder bump and the connected material is damaged. Accordingly, generation of the voids in the solder bump must be suppressed.
In contrast, in the electronic parts module mounting steps, it is normal that terminals of the electronic parts module are mounted on the wiring substrate under the condition that the flux or flux containing paste is coated on the surface of the wiring substrate, etc. Since the flux is dissolved by heat at the time of mounting, poisonous gas is generated, and the safety of operation must be assured. Also, if the halogen component remains as the residue of the flux, corrosion and migration of the wiring on the substrate are promoted and therefore the thorough cleaning is needed, which acts as one cause to increase the production cost.
Following several methods have been known as the solder mounting step.
First, the method of reflowing the solder under the reduced pressure or vacuum atmosphere is set forth in Patent Application Publication (KOKAI) Hei 4-220166, Patent Application Publication (KOKAI) Hei 5-211391, Patent Application Publication (KOKAI) Hei 6-29659, Patent Application Publication (KOKAI) Hei 7-79071, Patent Application Publication (KOKAI) Hei 7-170063, etc. As the equipment used in the solder reflow, in Patent Application Publication (KOKAI) Hei 4-220166, for example, such a mechanism is employed that the preheating furnace and the reflow main furnace are arranged via the gate, the exhaust system is provided to them respectively, and only nitrogen gas is introduced into the preheating furnace.
Second, the method of reflowing solder by using carboxylic acid is set forth in Patent Application Publication (KOKAI) Hei 6-190584, Patent Application Publication (KOKAI) Hei 6-267632, and Patent Application Publication (KOKAI) Hei 7-164141. The equipment used in this solder reflow has a configuration that introduces directly the inert gas, the reduction gas, the carboxylic acid solution, and the diketone solution into the heating zone and also foams the diketone. But this equipment has a mechanism that heats the substrate in the atmospheric pressure atmosphere without the gate and the exhaust system.
Third, the method of reflowing solder by positively using the reducing gas is set forth in Patent Application Publication (KOKAI) Sho 62-102546, Patent Application Publication (KOKAI) Hei 2-41772, Patent Application Publication (KOKAI) Hei 4-258737, Patent Application Publication (KOKAI) Hei 6-190584, and Patent Application Publication (KOKAI) Hei 6-326448.
However, since the oxidized layer formed on the deposited surface cannot be easily removed even if the soldering method applied under a reduced pressure atmosphere is employed, the formability of the bump is degraded. Also, the inventors of the present invention could not obtain the improved result when they examined the void-generation suppressing effect by merely producing the reduced pressure atmosphere.
Also, the void-generation suppressing effect could not be obtained at all by soldering using carboxylic acid.
In the meanwhile, in some cases the soldering applied in a hydrogen atmosphere is effective to reflow the solder bump at a high temperature. However, since the electrode pad acting as the underlying layer of the solder bump occludes the hydrogen and thus adhesiveness between the electrode pad and the underlying insulating layer is degraded, such a possibility is increased that the troubles such as peel-off of the electrode pad, etc. are caused. Further, it is set forth in Patent Application Publication (KOKAI) Hei 6-190584 that the hydrogen, etc. do not have substantially the reducing power for the oxidized layer at the temperature of less than 350xc2x0 C.
It is an object of the present invention to provide a semiconductor device manufacturing method containing a solder bump forming step which is able to form a solder bump without the use of flux not to generate voids in a solder layer and also does not need cleaning after the solder bump is shaped, and heating/melting process equipment employed in this method.
Also, it is another object of the present invention to provide an electronic parts mounting method containing a step which is able to simplify electronic parts module manufacturing steps by omitting the use of flux and at the same time improve the reliability of products and exert no influence upon the environment, and heating/melting process equipment employed in this method.
According to the present invention, in the step of forming solder bumps on the connected material or the step of mounting the solder of the electronic parts onto the connected material, the solder is heated/melted in the reduced pressure atmosphere containing the formic acid, or the solder is heated/melted in the reduced pressure atmosphere after the formic acid is coated on the surface of the solder.
Therefore, the oxidized layer on the surface of the connected material can be removed without leaving residue on the surface, and good surface shapes of the bumps or good shapes of the solidified solder can be achieved. In addition, generation of voids in the solder bumps and the soldering portion can be suppressed.
Furthermore, according to the present invention, since a phenomenon of embrittling the material to be connected does not occur, peel-off of the connected material can be prevented.
Besides, it is possible to omit the cleaning after the solder bumps are formed or the solder is solidified. However, if the formic acid remains on the surface of the solder, there is a possibility to reoxide the solder in the air. Therefore, it is preferable that, after the solder is heated/melted and then solidified, the heating of such solder should be held at the temperature of below the melting point of the solder and in excess of the boiling point of the formic acid. As a result, since the formic acid can be evaporated from the surface of the solder, reoxidation of the solder can be prevented.
If the formic acid is supplied previously to the surface of the solder before the solder is heated/melted, the reduction of the formic acid is gradually proceeded in the course of the gradual increase of the solder temperature. Therefore, the voids generated in the solder or the voids existing in the solder can escape gradually. As a result, since the voids disappear, the defective shape of the solder and the scattering of the solder are not caused.
According to the heating/melting process equipment of the present invention, since the formic acid recovering mechanism and the formic acid decomposing mechanism are provided on the exhaust side of the heating/melting chamber, scattering of the formic acid into the air can be prevented. Also, since the heating mechanism for heating the formic acid is provided to the portion that discharges the formic acid into the chamber, the droplet state of the formic acid on the surface of the solder can be suppressed. As a result, the formic acid is difficult to remain on the surface of the solder after the solder has been heated/melted, and thus reoxidation of the solder can be prevented.